The description of fn:base-uri starts "Returns the value of the base-uri URI property". Here "URI" is clearly superfluous and wrong. However, as so often happens with F&O, closer reading reveals the whole spec to be a mess. The Summary is not a summary. The spec is completely confused between the base-uri *property* of the node as defined in XDM, and the dm:base-uri *accessor* as defined in XDM. Much of what it says is actually a paraphrase of the XDM definition of the dm:base-uri accessor, but this is not made clear. The following suggested rewrite makes text that duplicates XDM into a Note:

Summary: Returns the base URI of a node.

The zero-argument version of the function returns the base URI of the context node: it is equivalent to calling fn:base-uri(.). This may result in an error being raised: if the context item is undefined [err:XPDY0002]XP; if the context item is not a node [err:XPTY0004]XP.

The single-argument version of the function behaves as follows:

1. If $arg is the empty sequence, the function returns the empty sequence.

2. Otherwise, the function returns the value of the dm:base-uri accessor applied to the node $arg. This accessor is defined, for each kind of node, in the XDM specification (see Section 5.2 base-uri AccessorDM).

<note>As explained in XDM, document, element and processing-instruction nodes have a base-uri property which may be empty. The base-uri property for all other node kinds is the empty sequence. The dm:base-uri accessor returns the base-uri property of a node if it exists and is non-empty; otherwise it returns the result of applying the dm:base-uri accessor to its parent, recursively. If the node does not have a parent, or if the recursive ascent up the ancestor chain encounters a parentless node whose base-uri property is empty, the empty sequence is returned. In the case of namespace nodes, however, the result is always an empty sequence -- it does not depend on the base URI of the parent element.</note>

See also fn:static-base-uri.